Oscilloscopes such as the DPO/DSA70000 Series Oscilloscopes available from Tektronix, Inc. of Beaverton, Oreg. use trigger circuits to provide stable waveforms for display, quickly capture abnormal behavior, etc. One type of trigger circuit is an “edge trigger” which triggers the oscilloscope when an input signal crosses a user-specified threshold voltage level in a user-specified direction. An edge trigger is typically implemented with a comparator. For example, as shown in FIG. 1, comparator 110 generates a trigger signal when an input signal exceeds a threshold voltage level.
However, input signals may not be single-ended as shown in FIG. 1, but instead may be differential. A differential signal is expressed as the difference between two complementary single-ended signals, each single-ended signal being referred to as one “side” of the differential signal. A conventional edge trigger circuit cannot trigger on a differential input signal because it only has one terminal for receiving an input signal, not two.
One solution to this problem is to apply only one side of the differential input signal to comparator 110, the rationale being that the one side is a single-ended version of the differential input signal having one half of the signal amplitude. However, this may result in inaccurate triggering when the differential input signal is not perfectly balanced. For example, suppose that the two sides of a differential input signal contain “common mode” noise, that is, noise which is present equally on both sides and thus not present in the difference signal because it is subtracted away. If only one side is applied to comparator 110 and comparator 110 generates a trigger signal in response to the common-mode noise, then that trigger signal is inaccurate because it reflects a characteristic not present in the differential input signal.
Another solution is to first convert the differential input signal into a single-ended signal and then apply that single-ended signal to comparator 110. This approach avoids the inaccurate triggering described above, however it only works for relatively low frequency input signals because all known methods of converting differential signals into single-ended signals either limit bandwidth or require a custom ASIC. For example, one such conversion method uses an op amp, however only relatively low bandwidth op amps are available such as the AD8000 available from Analog Devices, Inc. of Norwood, Mass. which has a −3 dB bandwidth of 1.5 GHz, much lower than the bandwidth of a comparator such as the 25706CP available from InPhi Corporation of Westlake Village, Calif. which has an operating bandwidth of 25 GHz. Furthermore, op amps inject noise onto the input signal which results in trigger jitter.
Yet another solution is to use two comparators to separately compare each side of the differential input signal to the threshold voltage level, and then combine the output signals of the two comparators to form the trigger signal. This approach avoids a differential-to-single-ended conversion, however the two output signals may transition at different times when the differential input signal is not perfectly balanced or when the threshold voltage level is set to other than the mid-point of the signals, and thus the combined trigger signal may have a piecewise transition which is unsuitable for triggering test and measurement instruments.
What is desired is trigger circuit capable of generating a trigger signal from a differential signal that operates at high bandwidth and with low jitter.